Printing plates for flexographic printing to be used for printing packaging materials, decorative building materials and the like have conventionally been produced by exposing a printing original plate made of a photosensitive resin to light imagewise to crosslink the resin in the exposed portion, and then washing off and removing the uncrosslinked resin in the unexposed portion. In recent years, however, printing plates produced by laser engraving have been spreading for the improvement in efficiency of printing plat production. In such printing plates, a relief image is formed directly on a printing original plate by the use of laser. In the process of producing a printing plate by laser engraving, projections and recesses are formed by decomposing an image-forming material in an irradiated portion through imagewise-irradiation of a printing original plate with a laser beam. During this process, a viscous residue is formed through the decomposition of the image-forming material in the laser-irradiated portion and some of the residue also scatters to the laser-unirradiated portion. Since the residue will cause a problem if it is left on a printing plate, it is removed from a printing plate by suction with a dust collector provided near a laser machine during the laser irradiation and/or by washing of the printing plate after the laser irradiation.
Conventionally known printing original plates for laser engraving include plates made from a resin composition prepared by incorporating a photopolymerizable compound and a photopolymerization initiator to a synthetic rubber or a natural rubber. Such a printing original plate, however, shows high tackiness because it contains a rubber as its major component and, therefore, a residue produced by laser irradiation is likely to be left adhered to the plate without being removed even if the sucking during the laser irradiation or the washing after the laser irradiation is performed. If a residue is left adhered to a laser-unirradiated portion (projecting portion) of a printing original plate, printing deficiency may occur because this portion is a part to which an ink is to be provided. Moreover, if a residue is left adhered to the bottom of a laser-irradiated portion (recessed portion) of the printing plate, the depth of a screen dot will decrease. If a residue is left adhered to a side surface of a recessed portion, the reproducibility of a screen dot will decrease. In both the events, decrease in resolution may be caused.
In order to cope with this problem, technologies to improve mechanical properties of a printing original plate by incorporating a laser-absorptive colored filler such as carbon black to a resin composition or by incorporating a colorless transparent filler such as silica microparticles to a resin composition so as to reduce the tackiness as a result have been proposed (cf. Japanese Patent Application Laid-Open (JP-A) No. 2004-533343). The method of incorporating a laser-absorptive filler such as carbon black, however, has a problem that since using a colored filler, the resin composition is made opaque, so that it becomes impossible to fully perform photo-curing by light irradiation. Further, while the method of incorporating a filler such as silica microparticles does not cause a problem like that caused in the case of incorporation of carbon black because the filler is colorless and transparent, it requires a large amount of filler in order to reduce the tackiness of a printing original plate sufficiently and it has a problem of remarkably impairing the moldability or the physical properties of a printing original plate. As mentioned above, the addition of a filler has some adverse effect on the moldability or the physical properties of a printing original plate. Therefore, a method by which the tackiness of a printing original plate can be reduced without adding a filler has been awaited to be developed.